A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these properties comprehensively, it is effective to make a steel material with a fine-grained structure; to this end, a number of manufacturing methods to obtain a steel material with a fine-grained structure have been sought. Further, by making the fine-grained structure, it is possible to manufacture a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements added is reduced.
As a method for making a steel sheet with a fine-grained structure, it is known to carry out a large rolling reduction especially in the subsequent stage of hot finish rolling (in any rolling mill to roll a steel sheet on downstream side when a plurality of rolling mills are aligned in parallel), deforming austenite grains greatly and increasing a dislocation density; and thereby to obtain fine-grained ferrite after rolling. Further, in view of facilitating the ferrite transformation by inhibiting recrystallization and recovery of the austenite grains, it is effective to cool a steel sheet to 600° C. to 750° C. as quickly as possible after rolling. In other words, subsequent to hot finishing rolling, it is effective to rapidly cool a steel sheet after the rolling, by arranging a cooling apparatus capable of cooling more quickly than ever before. In rapidly cooling a steel sheet after rolling in this way, it is effective to have a large volume of cooling water sprayed over the steel sheet per unit area, and to make a volume density of cooling water (sometimes referred to as “cooling water volume density”) large in order to enhance a cooling capability.
However, if the cooling water volume density is increased in this way, the water accumulated (i.e. retained water) on an upper surface of a steel sheet increases due to a relation between water supply and water discharge. By the increase of the retained water, the retained water reaches an upper surface guide disposed between the steel sheet and a cooling nozzle and having a hole that allows cooling water sprayed from the cooling nozzle to pass through, whereby so-called overflow can occur. The overflow sometimes causes troubles as follows.
(1) By making a thick layer of the retained water, jet pressure of the cooling water sprayed from the cooling nozzle decays. If the layer of the retained water becomes even thicker and reaches the cooling nozzle, the jet pressure decays more.
(2) In discharging the retained water, the retained water has contact with the upper surface guide and creates a flow resistance, whereby discharging capability degrades.
(3) Since it is difficult to control overflowed water, the water can flow into other areas and so on, which can cause unexpected problems.
Therefore, because of such troubles as above, there is a problem that high cooling capability cannot be exerted, and sometimes it is difficult to effectively have cooling water with a large volume density to spray to a steel sheet.
With regard to discharging water on an upper surface side of a steel sheet, techniques such as Patent Document 1 and 2 have been disclosed. In a cooling apparatus of a hot-rolled steel strip described in Patent Document 1, a hole is provided to an upper surface guide configured to supply cooling water by allowing the cooling water to pass through, and to overflow retained water. Also, in a cooling apparatus of a steel sheet described in Patent Document 2, a hole to supply cooling water to an upper surface guide and a slit to handle overflow are provided separately to allow retained water to discharge smoothly thereto inhibit degradation of cooling capability.